Transcutaneous electrodes are used for long term monitoring of electro-physiological phenomenon because they are easily retained for long periods, provide a good signal, and are amenable to attachment to difficult areas. One of the more common uses is for detecting the fetal electrocardiogram (ECG) and other heart beat signals during labor, where an electrode must be placed easily, remain firmly attached, and be atraumatic, especially on removal.
The prior art teaches the use of several types of electrodes for transcutaneous monitoring, including standard injection needles, practically all of which are administered to the patient substantially perpendicular to the skin, thus risking the danger of too deep a puncture, etc. To facilitate holding the transcutaneous electrode in place, especially in fetal monitoring, several adaptations have been proposed and/or used, including the surgical clip arrangement of Hon (Instrumentation of Fetal Heart Rate and Fetal Electrocardiography III. Fetal ECG Electrodes: Further Observations, Obstet. Gyn. 30:281,1967); the fish-hook type arrangement of La Croix (U.S. Pat. No. 3,580,242); the double and single spiral arrangements of Ruttgers and Hon (U.S. Pat. No. 3,750,650 and RE 28,990 respectively); and the barbed, straight needle arrangement of Junge (German Patent Application No. 74,19264).
The surgical clip and spiral electrodes are traumatic (the latter primarily from the torque created on application and must be inserted (and in the case of the surgical clip also removed) with a special tool, placing the surgeon's hand well away from the presenting part, and thus losing the "feel" of the situation, reducing physician confidence, and adding a possibility for error in placement. Since the physician is trained to palpate (i.e. feel) and use this modality to a great extent, this loss can be rather disconcerning.
The fish-hook electrode is also traumatic, especially on removal. The barbed needle electrode is significantly smaller than the others and offers a better opportunity for "feel", but its rough edges, i.e. the multiplicity of little barbs, also tend to cause trauma. Moreover, application thereof is difficult in the absence of an applicator tool, with the electrode often slipping away from the entry site when force is applied to effect skin penetration.